1. Field of the Invention
The invention relates to a method of manufacturing polysaccharide graft polymers which strongly absorb water and are thereby capable of swelling. The polymers are produced from a polysaccharide (as the grafting base) and unsaturated water-soluble monomers which contain carboxyl groups.
2. Description of the Background
Water absorbing polymers have numerous applications in the sanitary and hygiene areas, for example, as a water absorption medium in paper diapers and paper wipers, as tampons, hospital bedding, electrolyte thickeners in dry batteries, moisture conserving or water storing materials in agriculture, and as drying agents.
Suitable polymers are derivatized polysaccharides, generally grafted with water-soluble vinyl monomers. The polysaccharides employed include, for example, carboxymethyl-cellulose, hydrolyzed starch/acrylonitrile graft polymers, or acrylic acid/starch graft polymers. Other suitable water absorbing polymers include completely synthetic, weakly crosslinked polymers; e.g., crosslinked polyethylene oxide, or partially crosslinked polyacrylate salts. Of these polymers, carboxymethylcellulose and partially crosslinked polyethylene oxide have water absorption capacities, for fully deionized water ("FD"-water), which are only about 30 g per gram of polymer.
Partially saponified starch/acrylonitrile graft polymers, on the other hand, have high water absorption capacities, i.e., 300 to 500 g FD-water/g. These polymers can be prepared by grafting acrylonitrile on starch in aqueous suspension in the presence of Ce(IV) salts, with subsequent hydrolysis using aqueous NaOH or KOH, and possibly with purification by reprecipitation. This method requires the handling of acrylonitrile as a process material. Acrylonitrile is classified as hazardous. In addition, the monomers must be completely removed from the products by degassing which is particularly necessary if the products are to be used in the hygiene sector. Another consideration is that the hydrolysis of the nitrile groups is a technically difficult process, because it gives rise to highly viscous, water-swollen raw products having a doughy consistency and inferior water absorption capability. The water absorption capability can be improved only by tedious purification steps.
Accordingly, attempts have been made to synthesize water-absorptive graft polymers by direct grafting of water soluble monomers (e.g., acrylic acid and acrylamide) onto starch, in aqueous or alcoholic solution.
Japanese Pat. No. 40-43,408 describes a method of grafting unsaturated water-soluble monomers such as acrylic acid onto gelatinized starch, in aqueous solution in the presence of a water-soluble initiator system comprised of hydrogen peroxide and ascorbic acid. The method yields water soluble polymers with strong thickening action but low water absorption capacity (about 100 g FD-water/g).
It is also known, from Japanese Pat. No. 80-139,408, that graft polymerization of (meth)acrylic acid onto starch in aqueous solution, with initiation by water soluble peroxides, yields gel-like water-absorbing polymers with good water absorption capacity (&gt;200 g FD-water/g). In order to achieve this result, it is necessary to bring about a weak crosslinking, by incorporating small amounts of di- or trifunctional compounds (e.g., glycerine diglycidyl ether, N,N'-methylenebis-acrylamide, or 1,1,1-tris(hydroxymethyl)propane triacrylate.
The following difficulties are associated with polymerization of water soluble monomers in aqueous solution, with crosslinking, to produce particulate gels:
(1) The polymerization with crosslinking necessarily leads to a network of chains of low elasticity;
(2) The reaction products obtained are water-swollen gels, which are difficult to handle. The solid must be separated out by precipitation from the aqueous phase, yielding a clumped precipitate, which then must be dried and comminuted;
(3) Due to the swelling of the graft polymers, and high viscosity, it is not possible to intermix and stir the aqueous solutions, even at low solids contents of 10-20 wt. %. Therefore, it is not possible to carry out a controlled reaction in ordinary stirred apparatus;
(4) Using only water as a reaction medium, it is not possible to carry out effective graft polymerization of water soluble monomers onto starch, with formation of gel-containing graft polymers having the branched chain structure required for good water absorption capacity. Water soluble monomers such as acrylic acid or acrylamide polymerize in a solely water reaction medium with unavoidable side reactions leading to water soluble, non-grafted hompolymers, which have a thickening action but do not swell in water, and therefore detract from the water absorption capacity.
Measures to vary the degree of the graft polymerization can be employed in connection with the abovementioned aqueous solution polymerization technique. These include increasing the concentration of the grafting substrate, increasing the monomer concentration, and increasing the initiator concentration. However, the effects achievable by these measures are highly limited. When one increases the concentration of the grafting substrate, additional increases in the viscosity occur. Increasing the initiator concentration can lead to an uncontrollably high rate of polymerization, and low molecular weight.
The technique of water-in-oil suspension polymerization (inverse suspension polymerization) has been developed as an improved polymerization method for producing high molecular weight homo- and graft polymers in solid, fine particulate form.
This technique has proven to be very advantageous for producing graft polymers from polysaccharides such as starch, where the monomers used for the graft polymerization are water soluble. The success of the technique is attributable to the fact that the preconditions for effective graft polymerization are met (e.g., high concentrations of starch, monomers, and peroxide; and good stirrability of the suspensions). Inverse suspension polymerization for graft polymerization of water soluble monomers onto polysaccharides is described in German Pat. No. 28 40 010. The suspension polymerization is carried out semi-continuously. Starch, as the graft polymerization substrate, is suspended in petroleum ether containing a solvent-soluble, surface active agent such as sorbitan monooleate. An aqueous solution of the monomer which is prepared in a separate container is added, and the polymerization is begun by adding a radical initiator. The reaction is continued for 0.5 to 6 hr. The graft mixed polymer is isolated in the form of solid particles. The surface active agent is an essential feature of the method.
This technique has the following characteristics:
(1) The monomers are furnished in an aqueous solution prepared in a separate reaction vessel, along with a relatively high amount of a water soluble surface active agent which are then added to the organic phase. The surface active agent is regarded as necessary to attain homogeneous distribution of the aqueous phase in the organic phase. The so-called "semi-continuous" process requires an additional reaction vessel, which, when acrylic acid is being added as the monomer, must be equipped with a cooling apparatus to remove the heat of neutralization.
(2) Preferably, mixtures of ionic and nonionic surface active agents are used, in high amounts of from 0.5 to 12 wt. % based on the weight of the solvent.
Japanese Pat. No. 80-161,813 describes the production of a water-absorbing graft polymer by graft polymerization of acrylic acid onto a polysaccharide, in a gasoline-type hydrocarbon solvent, in the presence of an organophilic nonionic surface active agent and a water soluble initiator. For example, an aqueous phase comprising cornstarch, acrylic acid, NaOH, and sodium persulfate is dispersed in an organic phase comprised of n-hexane with 2 wt. % of sorbitan monostearate added, and polymerization is carried out for 6 hr at 60.degree. C., to yield a gel-like graft polymer with a water absorption capacity of 600 g FD-water/g.
European No. A2 0 036 463 and German OS No. 33 31 644 and OS No. 35 07 775 describe methods of inverse suspension polymerization of carboxyl- or carboxylate group containing water soluble monomers in the presence of small amounts of a crosslinking agent, for producing strongly water-absorbing, gel-like polymers. In all the examples given, the polymerization is carried out by a semi-continuous method involving relatively high apparatus costs. The aqueous monomer solution is prepared by neutralization of acrylic acid with an aqueous alkali- or ammonium hydroxide solution, with cooling, in a separate reaction vessel, and this solution is then added in portions to the organic phase. These methods differ in the types of surface active agents used. The parameter known as the "HLB value" (Hydrophilic-lipophilic balance value) is used to characterize the surface active agent. Lipophilic surface active agents have HLB values of 0 to 10, and hydrophilic surface active agents have values of 10 to 20.
In the European No. A2 0 036 463, an oil-soluble surface active agent with relatively high HLB value of 8 to 12 is used, with sorbitan monolaurate preferred. High formation of gels and clumps occurs in the polymerization. The products are distinguished by high water absorption capacities, in some cases on the order of 1,000 g FD-water/g.
In German OS No. 33 31 644 and OS No. 35 07 775, the aqueous monomer solution is added portionwise to the organic phase, which is hot and which contains an oil-soluble surface active agent with low HLB-value. The water absorption capacities of the resulting polymers are approximately 450-750 g FD-water/g.
It can be concluded from these patent publications that when nonionic surface active agents with high HLB values (thus high water solubility) are used in semicontinuous suspension polymerization, the result is polymers with a large particle size and high water absorption capacity, and with a strong tendency to form gels. On the other hand, when nonionic surface active agents with low HLB values are used, the result is polymers with small particle size and low water absorption capacity, and a low tendency to form clumps.
There continues to be a need for a simplified method of producing graft polymers having a small particle size and high water absorption capacity but low water-solubility, using polysaccharides and carboxyl- or carboxylate containing vinyl monomers. The graft polymers should precipitate directly in the polymerization, as a powder with high water absorption capacity and low tendency to separate out on vessel walls or to form clumps.